Move checker documentation out of cbmultimanager repo

The cluster monitor's checkers are (temporarily) documented at https://github.com/couchbaselabs/cbmultimanager/blob/master/doc/checkers.md. This is not where they're going to live long-term - possible places are:

• In the CMOS AsciiDoc
• In a KB article (likely managed by Support)
• Somewhere else?

We can decide on a permanent place later down the line, but for 0.1 at least we should move them somewhere that's accessible to users, even if we end up moving it again later.